Underground Power Underground Power TransmissionTransmission

IntroductionIntroduction• Since the loads having the trends towards growing density. This requires

the better appearance, rugged construction, greater service reliability and increased safety.

• An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.

• The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service.

• The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.

AdvantagesAdvantagesThe underground cables have several advantages such as,

– Better general appearance

– Less liable to damage through storms or lighting

– Low maintenance cost

– Less chances of faults

– Small voltage drops

Dis-advantageDis-advantage

The major drawback is that they have greater installation cost and introduce

insulation problems at high voltages compared with equivalent overhead

system.

Overhead vs. UndergroundOverhead vs. Underground

Transmission: Underground may be 4-20 times Overhead.

Sub transmission: Underground may be 4-20 times Overhead.

Distribution: Underground may be 2-10 times Overhead.

5

Construction of CablesConstruction of Cables

• Core or ConductorA cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminium or copper and is stranded in order to provide flexibility to the cable.

• InsulationThe core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.

• Metallic SheathA metallic sheath of lead or aluminium is provided over the insulation to protect the cable from moisture, gases or othes damaging liquids

Core

Belted paper

Lead sheath

Bedding

Single wire armoring

Overall Serving

BeddingBedding is provided to protect the metallic sheath from corrosion and from mechanical damage due to armoring. It is a fibrous material like jute or hessian tape.

ArmouringIts purpose is to protect the cable from mechanical injury while laying it or during the course of handling. It consists of one or two layers of galvanized steel wire or steel tape.

ServingTo protect armouring from atmospheric conditions, a layer of fibrous material is provided.

Properties of Insulating MaterialProperties of Insulating Material

The insulating materials used in cables should have the following properties

High resistivity.

High dielectric strength.

Low thermal co-efficient.

Low water absorption.

Low permittivity.

Non – inflammable.

Chemical stability.

High mechanical strength.

High viscosity at impregnation temperature.

Capability to with stand high rupturing voltage.

High tensile strength and plasticity.

Insulating Materials for Cables

• RubberIt can be obtained from milky sap of tropical trees or from oil products.It has the dielectric strength of 30 KV/mm.Insulation resistivity of 10 exp 17 ohm.cmRelative permittivity varying between 2 and 3.They readily absorbs moisture, soft and liable to damage due to rough handling and ages

when exposed to light.Maximum safe temperature is very low about 38 C

• Vulcanized India Rubber

It can be obtained from mixing pure rubber with mineral compounds i-e zinc oxide, red lead

and sulphur and heated upto 150 C.

It has greater mechanical strength, durability and wear resistant property.

The sulphur reacts quickly with copper so tinned copper conductors are used.

It is suitable for low and moderate voltage cables.

• Impregnated Paper This material has superseded the rubber, consists of chemically pulped paper

impregnated with napthenic and paraffinic materials. It has low cost, low capacitance, high dielectric strength and high insulation

resistance. The only disadvantage is the paper is hygroscopic, for this reason paper insulation is

always provided protective covering.

• Varnished Cambric This is simply the cotton cloth impregnated and coated with varnish. As the varnish cambric is also hygroscopic so need some protection. Its dielectric strength is about 4KV / mm and permittivity is 2.5 to 3.8.

• Polyvinyl chloride (PVC) This material has good dielectric strength, high insulation resistance and high

melting temperatures. These have not so good mechanical properties as those of rubber. It is inert to oxygen and almost inert to many alkalis and acids.

• XLPE Cables (Cross Linked Poly-ethene)

This material has temperature range beyond 250 – 300 C

This material gives good insulating properties

It is light in weight, small overall dimensions, low dielectric constant and high

mechanical strength, low water absorption.

These cables permit conductor temperature of 90 C and 250 C under normal and

short circuit conditions.

These cables are suitable up to voltages of 33 KV.

CLSSIFICATION OF CABLESCLSSIFICATION OF CABLES

• Low tension (L.T) ----- up to 1000VLow tension (L.T) ----- up to 1000V

• High tension (H.T) ----- up to 11, 000VHigh tension (H.T) ----- up to 11, 000V

• Super tension (S.T) ---- from 22KV to 33KVSuper tension (S.T) ---- from 22KV to 33KV

• Extra high tension (E.H.T) cables ------- from 33KV to 66KV Extra high tension (E.H.T) cables ------- from 33KV to 66KV

• Extra super voltage cables ------beyond 132KVExtra super voltage cables ------beyond 132KV

A cable may have one or more than one core depending upon the type of

service for application. It may be,

Single Core

Two Core

Three Core

Four Core

3- Core Cables3- Core Cables • Solid Type CablesSolid Type Cables

1. Belted Cables

In these cables the conductors are

wrapped with oil impregnated paper,

and then cores are assembled with filler

material. The assembly is enclosed by

paper insulating belt. These can be used for voltages up to 11KV or in some cases can be

used up to 22KV. High voltages beyond 22KV, the tangential stresses becomes an

important consideration. As the insulation resistance of paper is quite small along the layer,

therefore tangential stress set up, hence, leakage current along the layer of the paper insulation.

This leakage current causes local heating, resulting breaking of insulation at any moment.

2. Screened Cables

• These can be used up to

33kv but in certain cases

can be extended up to 66kv.

• These are mainly of two types H-type and S.L type cables

a. H-TYPE Cables: • Designed by H. Hochstadter.

• Each core is insulated by layer of impregnated paper.

• The insulation on each core is covered with a metallic screen which is usually of perforated aluminum foil.

• The cores are laid in such a way that metallic screen make contact with one another.

• Basic advantage of H-TYPE is that the perforation in the metallic screen assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated.

• The metallic screen increase the heat dissipation power of the cable.

a. S.L - Type: (Separate Lead)

• Each core insulation is covered by its own lead sheath.• It has two main advantages, firstly the separate sheath minimize the

possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath.

• The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore for greater care is required in manufacturing.

• Pressurized Type CablesPressurized Type Cables• In these cables pressure is maintained above atmosphere either by oil or by gas.• Gas pressure cables are used up to 275KV.• Oil filled cables are used up to 500KV.

• Oil Filled Cables• Low viscosity oil is kept under pressure and fills the voids in oil impregnated paper

under all conditions of varying load.• There are three main types of oil filled cables

a. Self-contined circular type

b. Self-contained flat type

c. Pipe Type cables

Pipe Type Cable

Sheath Channel Oil Filled 3-Core Oil filler Cable

Advantages of Oil Filled Cables

Oil filled cables have the following advantages over solid cables

• Greater operating dielectric stresses

• Greater working temperature and current carrying capacity

• Better impregnation

• Impregnation is possible after sheath

• No void formation

• Smaller size of cable due to reduced dielectric thickness

• Defect can easily be detected by oil leakage

• Gas Pressure CablesIn these cables an inert gas like nitrogen is used to exert pressure on paper dielectric to prevent void formation.

These are also termed as Compression cables They insulated cores similar to solid type The cable is inserted in a pressure vessel which may be a rigid steel pipe, commonly known

as pipe line compression cable. The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/ square meter

with a maximum pressure of 1.725 * 10 exp 6 N/ square meter.

• Compressed Gas Insulated Cables (GIC)

• In GIC cables high pressure sulphur hexaflouride (SF6), fills the small spaces in oil impregnated paper insulation and suppresses the ionization.

• Most EHV and UHV lines insulated with sulphur hexaflouride (SF6) gas are being used extensively for voltages above 132 KV up to 1200 KV.

• These cables are very popular for short lengths, river crossings and high way crossings.

• Advantages

Gas Insulated Cables have several advantages over oil filled cables,

• Efficient heat transfer hence can carry more current.

• Low dielectric loss and low capacitance

• SF6 gas is non-toxic, chemically stable and non-inflamable.

• Terminations of GIC cables are simpler and cheaper.

Laying of Underground Cables

• The reliability of underground cable network depends to a considerable extent upon proper laying.

• There are three main methods of Laying underground cables

a. Direct Laying

b. Draw in system

c. Solid system

Direct Laying• This method is cheap and simple and is most likely to be used in practice.• A trench of about 1.5 meters deep and 45 cm wide is dug.• A cable is been laid inside the trench and is covered with concrete material or bricks in

order to protect it from mechanical injury.• This gives the best heat dissipating conditions beneath the earth.• It is clean and safe method

Disadvantages• Localization of fault is difficult• It can be costlier in congested areas where

excavation is expensive and inconvenient.• The maintenance cost is high.

Draw in System

• In this conduit or duct of concrete is laid in ground with main holes at suitable positions along the cable route.

• The cables are then pulled into positions from main holes.• It is very high initial cost• Heat dissipation conditions are not good• This method is suitable for congested areas where excavation is expensive and

inconvenient• This is generally used for short lengths cable route such as in workshops, road

crossings where frequent digging is costlier and immposible

Solid System

• In this system the cable is laid in open pipes or troughs dug out in earth along the cable route.

• The troughing is of cast iron or treated wood

• Troughing is filled with a bituminous after cables is laid.

• It provides good mechanical strength

• It has poor heat dissipation conditions

• It requires skilled labour and favorable weather conditions

• It is very much expensive system

Grading of Cables

• Since the stresses are maximum at surface of the conductor or inner most part of the dielectric.

• The stress goes on decreasing as outer most layer is reached.

• Since the process of achieving the uniform electrostatic stresses on the dielectric of cables is known as Grading of cables.

• The unequal distribution of stresses is undesirable because,

if dielectric is chosen according to maximum stress the thickness of cable increases or either this may lead to breakdown of insulation.

• The following are the two main methods of grading

Capacitance grading Inter sheath grading

A power cable mainly consists of three components Conductor Insulation /Dielectric sheath

These are of high conductivity copper or aluminum wires touched together to form a required shape.

Copper and aluminum conductors are used as conductor materials.

TYPES OF MATERIALS USED IN INSULATION.

High resitivity. High dielectric strength. Low thermal co-efficient. Low water absorption. Low permittivity. Non – inflammable. Chemical stability. High mechanical strength. High viscosity at impregnation temperature. Capability to with stand high rupturing voltage. High tensile strength and plasticity.

One or three such paper insulated conductors are placed in a circular form and group is covered with impregnated paper again. Necessary filler is also covered.

TRENCHES

Cables are generally laid in the ground or in ducts in the underground distribution system. For this reason, there are little chances of faults in underground cables. However, if a fault does occur it is difficult to locate and repair the fault because conductors are not visible. Nevertheless, the following are the faults most likely to occur in underground cables

1) open circuit fault2) short circuit fault3)earth fault

• When there is a break in the conductor of a cable, it is called open circuit fault.

• The open circuit fault can be checked by megger. For this purpose, the three conductors of the 3-core cable at the far end are shorted and earthed.

• The resistance between each conductor and earth is measured by a megger and it will indicate zero resistance in the circuit of the conductor that is not broken.

• However, if the conductor is broken, the megger will indicate infinite resistance in its circuit.

• When two conductors of a multi-core cable come in electrical contact with each other due to insulation failure, it is called a short circuit fault.

• Again, we can seek the help of a megger to check this fault.

• For this purpose the two terminals of the megger are connected to any two conductors.

• If the megger gives zero reading, it indicates short circuit fault between these conductors.

• The same steps is repeated for other conductors taking two a time.

• When the conductor of a cable comes in contact with earth, it is called earth fault or ground fault.

• To identify this fault, one terminal of the megger is connected to the conductor and the other terminal connected to earth.

• If the megger indicates zero reading, it means the conductor is earthed. The same procedure is repeated for other conductors of the cable.

SF6 Filled Cable

XLPE cable

Gas pressure cables.(A) External pressure cables.(B) Internal pressure cable.

(a) High pressure gas filled cable.(b) Gas cushion cable.(c) Impregnated pressure cable

Oil filled cables.(A) Single core oil filled cables used up to 132 KV.(B) Three core oil filled cables used up to 66 KV.

EXTRA SUPER VOLTAGE CABLE:-

• The purpose of the external layer in insulated power cables is to provide mechanical protection against the environment during the installation and operation of the power cable.

• Currently, materials commonly used as the external layer for extruded power cables include PVC and polyethylene.

• These materials are used for their ability to withstand the cable operating temperature.